AF: 300m master brake cylinder 2-ports (can I block one?) | Allpar Forums
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300m master brake cylinder 2-ports (can I block one?)

Discussion in 'LH: Large Cars, 1993-2004' started by MoPar~Man, Feb 6, 2019.

  1. MoPar~Man

    MoPar~Man Active Member

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    Both rear brake lines have developed leaks and I need to block them somehow before I repair them. The master cylinder has two ports - is that one for front and one for rear wheels? If so, if I remove the rear line and plug the port, will fluid still try to flow through the front port, into the ABS thing, and somehow still find a path to the rear lines?
     
  2. valiant67

    valiant67 ...

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    The lines should be diagonally split, meaning the RF and LR are on one port and LF and RR on the other. There is no safe way to do what you want.
     
  3. MoPar~Man

    MoPar~Man Active Member

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    I don't think that's how the brake lines are connected on "modern" cars. I know that's how it was back in the 60's and 70's. On the 300m (probably all LH cars) the two lines from the master cylinder go to the ABS unit, and they come out as 4 separate lines from the unit. Now how they are plumbed inside the unit - I don't know.
     
  4. MoPar~Man

    MoPar~Man Active Member

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    Yea, I had a look at the manual and they mention "diagonally-split" about 1/2 dozen times.The lines in the engine area looked ok, but where they made the bend to run under the car is where they look nasty, all the way to the back. So I cut them off at the bend (heh) and put some fittings on them at that point. I created a "bubble-flare" instead of the double or inverted flare and used a matching coupler, then I took a short length of pre-made pipe, bent it in a U, and joined them together to effectively close off the rear lines. Bled the air out at the U, and now I've just got front brakes. Probably should bleed the front, might get to that at some point. Takes a bit more effort to stop. When it warms up I'll run new lines all the way to the back. A few years ago I bought a set of flex lines from Rockauto when they were like a few buck each on clearance, so I'll be using those. The only thing I'm not sure if I'll be able to salvage are the "proportioning valves" that are supposedly in the lines right where the flex lines are. If I can't get them undone then I won't use them.
     
  5. Tomguy

    Tomguy Well-Known Member

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    I think I paid like $20 for new lines when I did mine. I did the single-flare as well, that's what the 300M needs for all the lines. I did 3 of the 4 lines, the right front hard line looked fine so I didn't touch it. The proportional valves are aluminum and the fittings that screw in are brass so you won't have a problem undoing those connections. I also bought all 4 flex lines for like $5-10 each likely at the same time you did from RockAuto.
     

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